Problem: On the first 3 tests of his chemistry class, Omar got an average score of 82. What does he need on the next test to have an overall average of 84?
Explanation: Let his score on the next test be $x$ The sum of all of his scores is then $3 \cdot 82 + x$ The same sum must also be equal to $4 \cdot 84$ Solve: $x = 4 \cdot 84 - 3 \cdot 82 = 90$.